Lyme borreliosis is a prevalent vector-borne disease in the United States, Europe, and parts of Asia, caused the infection with the bacterial pathogen Borrelia burgdorferi. Despite several decades of intense studies, many fundamental aspects of pathogen biology and infection remain largely unknown. B. burgdorferi infection can be difficult to diagnose, and a vaccine to prevent disease in humans is currently unavailable. The application will study how a select set of unique borrelial gene-products BB0323, BBA52 and BBA57, that play important roles in the spirochete infection cycle, contribute to specific aspects of microbial biology and pathogenesis and also assess their potential utility as Lyme disease vaccines. We have four major goals. First, we will examine how a critical virulence determinant of B. burgdorferi, BB0323 undergoes two-step proteolytic processing that ultimately produce distinct N- and C-terminal polypeptides, which play specific roles in cell fission and infectivity. Second, we have discovered that BBA52 is critical for pathogen transmission from ticks to mice and that the protein binds to a receptor in ticks. We propose to study a potential function of BBA52 in vector-pathogen interaction and its role in B. burgdorferi persistence and transmission through ticks. Third, we show that BBA57, a borrelial gene-product important for early spirochete infection in mice, is a major trigger of Lyme arthritis. Our studies will dissect detail molecular and cellular mechanisms of how BBA57 triggers host inflammatory responses and also facilitates spirochete infection. Finally, the proposed study will explore the identification f new vaccine candidates as well as New castle disease virus (NDV) vector-based immunization strategies against Lyme disease. As NDV appears to be safe in humans and is currently in use for therapeutic treatment of cancer patients, the proposed study will establish a foundation for it utility as a possible vaccine vector against human Lyme disease. In sum, these studies will contribute to the understanding of the intriguing biology and infectivity of B. burgdorferi and hel develop novel preventive and therapeutic measures to combat infection.